Voltage sensing and switching circuit

ABSTRACT

A circuit to sense variations in alternating voltage applied to a load and to interrupt power flow to the load on reduction of the applied voltage, including a rectifier to rectify the alternating voltage, a voltage regulator to apply a constant level part of the rectified voltage as the power supply to a Schmitt trigger, part of the rectified voltage controlling conductivity of a transistor that provides the input signal to the Schmitt trigger according to variations in the rectified alternating applied voltage, a Zener diode to reference the Schmitt trigger at a voltage slightly below the voltage applied to the transistor controlling the trigger, a Zener diode at the output of the trigger to control the presence or absence of signals from the Schmitt trigger to a further transistor that controls the gate of a triac connected in the power line to the load, and the triac being gated to a non-conductive state to block power flow to the load upon reduced alternating applied power. Said circuit also can be used to sense variations in direct current voltage applied to a load and to interrupt power flow to the load on reduction of the applied voltage, in which circumstance no rectifier is used and the direct current applied voltage is utilized in the same manner as the rectified voltage when alternating voltage variations are being sensed.

United States Patent [191 Frantz et al.

[54] VOLTAGE SENSING AND SWITCHING CIRCUIT [75] Inventors: Richard J.Frautz, Cheshire; Alton R. Morris, Rocky Hill, both of Conn.

[73] Assignee: Arrow-Hart, lnc., Hartford, Conn.

[22] Filed: Aug. 31, 1971 [21] Appl. No.: 176,587

[52] US. Cl. ..3l7/3l, 307/235, 307/290, 323/9 [51] Int. Cl. ..I-I03k3/295, H0211 3/08 [58] Field of Search.......307/290, 23 S; 323/22 T, 22SC, 323/24, 4, 9; 317/27, 31; 328/147 Wireless World July 1970; Pg. 316

451 March 6, 1973 Primary ExaminerGerald Goldberg Attorney-Davis, Hoxie,Faithful] & l-lapgood 57 ABSTRACT A circuit to sense variations inalternating voltage applied to a load and to interrupt power flow to theload on reduction of the applied voltage, including a rectifier torectify the alternating voltage, a voltage regulator to apply a constantlevel part of the rectified voltage as the power supply to a Schmitttrigger, part of the rectified voltage controlling conductivity of atransistor that provides the input signal to the Schmitt triggeraccording to variations in the rectified alternating applied voltage, aZener diode to reference the Schmitt trigger at a voltage slightly belowthe voltage applied to the transistor controlling the trigger, a Zenerdiode at the output of the trigger to control the presence or absence ofsignals from the Schmitt trigger to a further transistor that controlsthe gate of a triac connected in the power line to the load, and thetriac being gated to a non-conductive state to block power flow to theload upon reduced alternating applied power. Said circuit also can beused to sense variations in direct current voltage applied to a load andto interrupt power flow to the load on reduction of the applied voltage,in which circumstance no rectifier is used and the direct currentapplied voltage is utilized in the same manner as the rectified voltagewhen alternating voltage variations are being sensed.

7 Claims, 2 Drawing Figures VOLTAGE SENSING AND SWITCHING CIRCUIT Thisinvention is directed to a circuit that senses small variations ofpredetermined line voltage applied between two terminals to a loadconnected therebetween, and that interrupts the flow of power from theterminals to the load upon the line voltage dropping a small incrementbelow its rated value. Power again is applied to the load upon theapplied voltage rising to its predetermined value.

Many electrical devices are designed to operate at a rated voltage, anda reduction from this rated voltage can cause damage to the electricaldevice. An example of such a device is the coil circuit for a contactorof an electrical motor, with a reduced voltage to the coil circuitcausing damage to both the contactor and motor. By use of the presentinvention, small reductions in voltage can be sensed and responded to,to interrupt power flow to a load such as the exemplary coil circuituntil the applied voltage goes back to its rated or predetermined value.

In the present invention, the alternating voltage across the twoterminals is rectified, and a portion of the rectified voltage which isvariable in proportion to variations in the alternating voltage isapplied to control the conductivity of a transistor which in turnprovides control input signals to a Schmitt trigger circuit. The Schmitttrigger has a constant level supply voltage derived by a voltageregulator from the rectified voltage, and means are provided toreference the Schmitt trigger at a sufficiently high voltage level toallow small percentage variations in the applied alternating voltage tocontrol the operation of the Schmitt trigger. Means are provided at theoutput of the Schmitt trigger to result in the presence or absence of anoutput signal depending on the operative configuration of the triggercircuit, and the presence of an output signal controls the conductivityof a transistor that in turn controls the gate of a solid state switchconnected in the line of power flow to the load. Small reductions ofalternating voltage from the normal predetermined value operate theSchmitt trigger to render the solid state switch nonconductive to blockpower flow to the load.

Objects and advantages of the present invention include the provision ofa voltage sensing and switching circuit which is sensitive to smallpercentage variations in applied voltage, which provides the presence orabsence of a control signal to a device controlling power flow to theload dependent upon the value of applied voltage, which provides preciseswitching control, and which derives its power supply from the appliedvoltage.

These and other objects and advantages of the invention will be apparentupon consideration of the following description and attached drawing, inwhich:

FIG. 1 illustrates the invention in block form; and,

FIG. 2 is a circuit diagram of the invention illustrating the voltagesensing and switching circuitry shown in block form in FIG. ll.

FIG. 1 shows an alternating potential applied across terminals 1 and 2,suitable for energizing a load 3 which may be any one of various typesrequiring voltage protection. The sensing and switching circuitry ofFIG. I is designed to allow power to flow to load 3 from terminals l and2 when a predetermined alternating voltage level is present at saidterminals, and to block power flow to load 3 when the alternatingvoltage level at the terminals drops below its predetermined value.Terminal 2 may be at ground potential or may be another phase line of amultiphase system.

Referring to FIG. 2, an alternating voltage applied between theterminals 1 and 2 is lowered by voltage dropping resistor 4 and isrectified by diode 5. The lowered pulsating direct current voltage atjunction 6 is then filtered to take out the pulsations by capacitor 7extending between junction 6 and terminal 2. The voltage at junction 6also is further dropped by voltage dropping resistor 8, and Zener diode9 connected between resistor 8 and terminal 2 acts as a voltageregulator to maintain a constant level of direct current voltage atjunction 10. The voltage at junction 10 is a portion of the rectifiedvoltage at junction 6 and is not effected by variations in the voltageat junction 6 occurring due to variation in the alternating voltagebetween terminals 1 and 2.

The voltage at junction 6 is also dropped through resistance voltagedivider 11 extending between junction 6 and terminal 2, and tap 12 whichis variable in position along the voltage divider takes off a portion ofthe rectified voltage at junction 6. The voltage on tap 12 will vary inproportion to variations in the alternating voltage between terminals 1and 2, and tap 12 thus serves to sense these variations.

Shown enclosed within dotted lines in FIG. 2 is a Schmitt triggercircuit S having input transistor 13 and output transistor 14. Theemitters 13c and 143 of these transistors are connected to one another,and the common connection is then connected to emitter resistance 15.The base 13b of transistor 13 is the input of the Schmitt triggercircuit and the output of the Schmitt trigger is taken on line 25connected to collector Me of transistor 14. The supply voltage to theSchmitt trigger circuit is obtained from the constant voltage level atjunction 10 and is present on line 16 extending from said junction. Thesupply voltage is thus self-contained within the circuit of the presentinvention, and no exterior supply source of direct current voltage isrequired. Collector resistances l7 and 18 are connected between supplyline 16 and the collectors 13c and Me of the transistors 13 and 14,respectively. Resistance 19 is connected between collector 13c oftransistor 13 and base 14b of transistor 14, and resistance 20 isconnected between base 14b of transistor 14 and junction 21 at the lowerend of common emitter resistance 15.

The Schmitt trigger operates in a manner whereby one of its twotransistors is conductive when the other is not, and the presence orabsence of an appropriate input signal determines which transistor isconducting. Resistances l9 and 20 provide the regenerative feedback ofthe trigger. A triggering input signal will render transistor 13conductive and transistor 14 non-conductive, and an insufficient inputsignal will render transistor ll3 non-conductive and transistor 14conductive. The Schmitt trigger will provide a high level output signalwhen transistor 13 conducts and a low level output signal whentransistor 14 conducts. As is common in a Schmitt trigger, its inherenthysteresis results in one voltage level of input signal operatingtransistor 13 from a conductive to a non-conductive state, and aslightly higher voltage level of input signal being required to operatetransistor 13 from its non-conductive state back to its conductivestate.

Tap 12 of voltage divider 11 is connected to the base 22b of transistor22, and capacitor 23 extending between base 22b and terminal 2 filtersout transient and ripple voltages which may be present at base 22b.Collector 220 of transistor 22 is connected to the supply voltagepresent on line 16, and emitter 22e of transistor 22 is connected to thebase 13b of the transistor 13 of the Schmitt trigger. Emitter 22e oftransistor 22 accordingly supplies the input signal to the Schmitttrigger circuit. Transistor 22 will be rendered highly conductive ormuch less conductive according to small changes in the signal on itsbase 22b, with its highly conductive condition providing a sufficientemitter current to turn on transistor 13 and turn off transistor 14 ofthe Schmitt trigger, and its less conductive condition providing for theturning off of transistor 13 and the turning on of transistor 14 of theSchmitt trigger. The signal to base 22b of transistor 22 varies inproportion to variations in voltage between terminals 1 and 2 asreflected at tap l2, and transistor 22 therefore will control theoperation of the Schmitt trigger according to variations in thealternating voltage between terminals 1 and 2. Transistor 22 alsoprovides a high impedance at the input of the Schmitt trigger to preventexcessive current flow and voltage drop through the upper part ofvoltage divider 11 and into the Schmitt trigger under normalpredetermined voltage conditions at terminals 1 and 2. These conditionswould otherwise occur in the absence of transistor 22 to disrupt theoperation of the sensing circuitry. Transistor 22 further allows smallchanges in voltage at tap 12 to result in sizable changes inconductivity of transistor 22 to provide thereby a precise triggering ofthe Schmitt trigger.

Zener diode 24 is connected between junction 21 and terminal 2, and hasthe function of referencing the Schmitt trigger at a fixed voltage abovethat of terminal 2 and only slightly lower than the voltage normallypresent at tap 12 for a predetermined line voltage normally appliedbetween terminals'l and 2. Zener diode 24 thereby greatly increases thesensitivity of the Schmitt trigger, so that the trigger transistors willchange their operating configurations upon a small percentage variationin the alternating voltage normally at 1 and 2. Without diode 24, apercentage deviation of approximately fifty percent would be requiredacross voltage divider 11 in order to operate the Schmitt trigger, sincethe Schmitt trigger would be turning off with about one-half volt at itsinput, and would be turning on with about 1 volt at its input. Withdiode 24, the

one-half volt differential to operate the Schmitt trigger will be inrelation to a much higher voltage that can be established at tap 12, andthis differential therefore will represent a small incremental change inthat high voltage.

The voltage on tap 12 may be varied by varying the position of the tapon voltage divider 1 1, thereby allowing fine tuning of the circuit.

Zener diode 26 is connected between the output line 25 of the Schmitttrigger and the base 27b of transistor 27. When transistor 14 of theSchmitt trigger is conducting, an insufficient voltage will be presentat collector 140 of transistor 14 to break down the diode, and

no output signal from the Schmitt trigger will pass across the diode.When transistor 14 is not conducting, however, a sufficient voltage willbe present at collec tor to cause an output signal from the Schmitttrigger through the diode to the base 271) of transistor 27. Diode 26therefore provides a clear presence or absence of output signal from theSchmitt trigger to transistor 27 depending upon the operativeconfiguration of the trigger. If diode 26 is not present, the Schmitttrigger will provide either a high or low output signal to transistor 27depending upon the operative configuration of the trigger.

The collector 270 of transistor 27 is connected to collector resistance29 which is in turn connected to the constant supply voltage on line 16.The emitter 27e of transistor 27 is connected to emitter resistance 30which in turn is connected to terminal 2. Resistance 28 is connectedbetween base 27b of transistor 27 and terminal 2, and provides a leakagepath to prevent leakage currents obtaining false triggering oftransistor 27 when transistor 14 is conducting. Emitter 27c oftransistor 27 also is connected to gate 31g of triac 31. Triac 31 isconnected between load 3 and terminal 2, and gating signals to its gate31g control the conductivity of triac 31 and accordingly the flow ofalternating current power to load 3. When a gating signal is appliedfrom transistor 27 to the triac gate 31g, the triac will conduct, andthe absence of such a signal will render the triac non-conductive.

Turning to the operation of the present invention, the normalpredetermined voltage between terminals 1 and 2 will establish arectified voltage at tap l2 and a current at base 22b of transistor 22sufficient to render transistor 22 highly conductive. Emitter 22c oftransistor 22 will then provide a triggering input signal to the Schmitttrigger to render its input transistor 13 conductive and its outputtransistor 14 non-conductive. There will then be a high enough voltageon collector 140 of transistor 14 to cause output current flow from theSchmitt trigger across Zener diode 26 and to the base 27b of transistor27. Transistor 27 will conduct, and emitter 272 will apply a gatingsignal to gate 31g of triac 31. Triac 31 will be rendered conductive,and power will flow between terminals 1 and 2 through load 3. If thenormal predetermined voltage between terminals 1 and 2 should drop belowrated value by a small increment, determined by the voltage at junction21, the circuit constants of the Schmitt trigger, and the setting of tap12 on voltage divider 11, the current at base 22b of transistor 22 willrender transistor 22 much less conductive so that its emitter 22c willnot provide a strong enough signal to the input of the Schmitt triggerto maintain input transistor 13 in a conductive state. Transistor l3accordingly will not conduct and transistor 14 then will becomeconductive. The voltage at collector 14c of transistor 14 will now beinsufficient to break down Zener diode 26, and no output signal will beprovided from the Schmitt trigger to base 27b of transistor 27.Transistor 27 will not conduct, no signal will be provided from emitter27e to gate 313 of triac 31, and triac 31 will not conduct. The loweredalternating voltage between terminals 1 and 2 accordingly has actedthrough the circuitry of the present invention to block power flow toload 3. When the alternating voltage between terminals 1 and 2 againrises, the circuitry corresponding to the numbering of FIG. 2, are asfollows:

Resistors (Ohms) Capacitors 4-5K,3watts 7- lSmf. 8 5K, 3 watts 23 0.22mf.

ll 460K, 0.5 watts l5 1K, 0.5 watts l7 56K, 0.5 watts Zener Diodes18-33K, 0.5 watts 9-32 v. l9 270K, 0.5 watts 24 16 v. 20 390K, 0.5 watts26 24 v. 28 K, 0.5 watts 29 10K, 0.5 watts Transistors 30 lK, 0.5 watts13,14, 22, 27 Fairchild 2N3569 Triac 31 Hudson II 14399 Source 120volts, 60 cycle A.C.

The foregoing description illustrates the principles of the inventionapplied to an individual embodiment, and the invention is to beinterpreted in a scope consistent with the spirit of its principles.

What is claimed is:

1. An undervoltage sensing circuit having first and second terminals,comprising:

a Schmitt trigger circuit having an input first transistor and an outputsecond transistor, the emitter of said transistors being connected toone another and to a common emitter resistance;

means for applying a constant level portion of an applied voltage acrossthe first and second terminals as the supply voltage to said Schmitttrigger circurt;

means for deriving a portion of said applied voltage,

which portion is variable in proportion to variations in said appliedvoltage;

means for applying an input signal to said Schmitt trigger of a valuedetermined by said derived variable portion of said applied voltage;

means including a zener diode connected between said Schmitt triggercircuit emitter resistance and said second terminal for referencing saidtrigger circuit at a voltage level intermediate said derived variableportion of said applied voltage and the voltage at said second terminalto increase the sensitivity of said Schmitt trigger circuit;

said Schmitt trigger circuit providing a high level output signal at apredetermined level of applied voltage across said first and secondterminals and providing a low level output signal at a lower level ofapplied voltage across said first and second terminals.

2. An undervoltage sensing circuit as claimed in claim 1 wherein:

said means for deriving a portion of said applied voltage comprises avoltage dividing resistance means.

3. An undervoltage sensing circuit as recited in claim 1 wherein:

said input signal applying means includes a fourth transistor having itscollector connected to said constant level applying means and itsemitter connected to the base of said input first transistor of saidSchmitt trigger circuit, and a voltage divider and tap for applying saidderived variable portion of said applied voltage to the base of saidfourth transistor. 4. An undervoltage sensing circuit as recited inclaim 1 including:

threshold means coupled to said output second transistor for conductingwhen said Schmitt trigger produces a high level output signal; and gatemeans coupled to said threshold means which is enabled by said thresholdmeans. 5. An undervoltage sensing circuit as recited in claim 4 wherein:

said threshold means includes a third transistor, andv a zener diodecoupled to the collector of said output second transistor and to thebase of said third transistor, and said gate means includes a triachaving its gate coupled to the emitter of said third transistor. 6. Anundervoltage sensing circuit as recited in claim 5 including:

a leakage resistance connected between the base of said third transistorand said second terminal. 7. An undervoltage sensing circuit as recitedin claim 5 wherein:

said undervoltage sensing circuit is electrically connected in parallelwith a load, one side of said triac being connected to said load, theother side of said triac being connected to said second terminal, saidload also being connected to said first terminal.

1. An undervoltage sensing circuit having first and second terminals,comprising: a Schmitt trigger circuit having an input first transistorand an output second transistor, the emitter of said transistors beingconnected to one another and to a common emitter resistance; means forapplying a constant level portion of an applied voltage across the firstand second terminals as the supply voltage to said Schmitt triggercircuit; means for deriving a portion of said applied voltage, whichportion is variable in proportion to variations in said applied voltage;means for applying an input signal to said Schmitt trigger of a valuedetermined by said derived variable portion of said applied voltage;means including a zener diode connected between said Schmitt triggercircuit emitter resistance and said second terminal for referencing saidtrigger circuit at a voltage level intermediate said derived variableportion of said applied voltage and the voltage at said second terminalto increase the sensitivity of said Schmitt trigger circuit; saidSchmitt trigger circuit providing a high level output signal at apredetermined level of applied voltage across said first and secondterminals and providing a low level output signal at a lower level ofapplied voltage across said first and second terminals.
 1. Anundervoltage sensing circuit having first and second terminals,comprising: a Schmitt trigger circuit having an input first transistorand an output second transistor, the emitter of said transistors beingconnected to one another and to a common emitter resistance; means forapplying a constant level portion of an applied voltage across the firstand second terminals as the supply voltage to said Schmitt triggercircuit; means for deriving a portion of said applied voltage, whichportion is variable in proportion to variations in said applied voltage;means for applying an input signal to said Schmitt trigger of a valuedetermined by said derived variable portion of said applied voltage;means including a zener diode connected between said Schmitt triggercircuit emitter resistance and said second terminal for referencing saidtrigger circuit at a voltage level intermediate said derived variableportion of said applied voltage and the voltage at said second terminalto increase the sensitivity of said Schmitt trigger circuit; saidSchmitt trigger circuit providing a high level output signal at apredetermined level of applied voltage across said first and secondterminals and providing a low level output signal at a lower level ofapplied voltage across said first and second terminals.
 2. Anundervoltage sensing circuit as claimed in claim 1 wherein: said meansfor deriving a portion of said applied voltage comprises a voltagedividing resistance means.
 3. An undervoltage sensing circuit as recitedin claim 1 wherein: said input signal applying means includes a fourthtransistor having its collector connected to said constant levelapplying means and its emitter connected to the base of said input firsttransistor of said Schmitt trigger circuit, and a voltage divider andtap for applying said derived variable portion of said applied voltageto the base of said fourth transistor.
 4. An undervoltage sensingcircuit as recited in claim 1 including: threshold means coupled to saidoutput second transistor for conducting when said Schmitt triggerproduces a high level output signal; and gate means coupled to saidthreshold means which is enabled by said threshold means.
 5. Anundervoltage sensing ciRcuit as recited in claim 4 wherein: saidthreshold means includes a third transistor, and a zener diode coupledto the collector of said output second transistor and to the base ofsaid third transistor, and said gate means includes a triac having itsgate coupled to the emitter of said third transistor.
 6. An undervoltagesensing circuit as recited in claim 5 including: a leakage resistanceconnected between the base of said third transistor and said secondterminal.